Measurements have been made of viscosity and temperature for the 1971 and older lavas. Viscosities were estimated by two complementary methods: the classical measurements of the flow rate, and a ballistic method, in which the apparent viscosity of the surface layers was deduced from the depth of penetration of a steel spear, perpendicularly injected into the lava flow. Laboratory measurements on lava samples have been made with rotating cylinder viscometers. The principal result of the work indicates that between the bocca and the flow front the flow behaviour, and consequently the viscosity may vary considerably from lava to lava, even though the original magma varies little in composition. It is suggested that the different types of flow arises from the fact that the overall viscosity depends not only on the temperature and composition of the residual vitreous phase, but to an even greater extent on the lava's crystal and bubble content. The devitrification, which is a determinant factor of the rheological properties, is in turn highly influenced by the temperature of eruption and the rate of cooling of the lava.